Not Applicable
Not Applicable
The present invention relates generally to a method for relieving the tracker load in dense moving target indication (MTI) environment, and more particularly to a method for automatic association of moving targets indications from entities traveling along known routes.
Radar is an electromagnetic sensing system used for detecting, locating, tracking and identifying objects of various kinds at considerable distances. It operates by transmitting electromagnetic energy towards objects (targets), and observing the echoes returned from them. The targets include aircrafts, ships, spacecrafts, automotive vehicles and other objects that may reflect at least a portion of electromagnetic energy back to a receiver. For a typical radar, a narrow beam of electromagnetic wave is radiated from an antenna to scan and search a region where targets are expected. When a target is illuminated by the beam, it intercepts some of the radiated beam and reflects a portion back towards the receiver. By measuring the time duration from radiating the electromagnetic beam to receiving the reflected portion, the range, that is, the distance between the target and the radar, can be determined. In addition, the angular direction (bearing) of the target can also be detected from experiencing the phase of the received signal and the boresight direction in which the antenna points. Through measurement of target location at successive instants of time, the track of a target can be determined.
Many targets, including stationary and moving objects, are operative to reflect electromagnetic energy. To detect moving targets, particularly in a military environment, contemporary radar systems can discriminate the moving targets from the stationary targets. It is known that, for a moving target, the returned signal has a frequency shift (wavelength change) from the signal radiated thereon. This is the Doppler frequency shift known in the art. By measuring the Doppler frequency shift, the moving targets can be detected and discriminated from the stationary targets (such as land or sea clutter). Radar systems that detect moving targets by measuring Doppler frequency shift are referred as the moving-target indication (MTI) radars.
In addition to the range and directional angle, MTI radars are also operative to track an object by measuring the velocity (frequency) or wavelength at successive instants of time. Usually targets are moving at different speeds, so each target will have a different Doppler frequency (wavelength) shift. Therefore, the measurement of the Doppler shift of each target will allow multiple targets to be differentiated from each other. In some instances, there may be hundreds of moving targets which must be monitored at any given time. In such a dense, moving-target indication environment, it may be difficult to track each moving target within the field of view. Once the MTI radar is overloaded, the track of each moving target may be incomplete and confusing; and consequently, it is difficult to characterize or differentiate from other moving targets. Therefore, it is desirable to provide a method to facilitate the association of received radar data to corresponding targets, thus mitigating the computation load of the radar.
The present invention provides a method for facilitating association of moving target indications from received radar data. The method comprises the following steps. A moving target indicator radar is used to detect a plurality of moving target indication data. The moving target indication data proximate to each identified route in the volume surveillance by the radar is selected and presented in a distance-time coordinate, such that each selected moving target indication data has a unique distance value and a unique time value. The time value is the time at which the MTI was observed. The distance value is the distance the MTI is from some arbitrary fixed point along the route. All selected moving target indication data proximate to a given route are then transformed from the distance-time coordinate to a slope-intercept coordinate, such that co-linear moving target indication data in the distance-time coordinate are transformed into a plurality of points superposed together with an identical slope value and an identical distance intercept value. All superposed points at each slope-intercept curves point to multiple observations of a single vehicle. They are mapped back to the distance-time coordinate, and the moving target indication data corresponding to the superposed points are thus associated.
In the above method, the step of selecting the moving target indication data further comprises predefining a proximity of the identified route and filtering the moving target indication data located beyond the proximity. For example, a proximate distance to the identified route can be predetermined based upon the spatial observation error of the MTI radar. The moving target indication data spaced from the identified route by a distance larger than the proximate distance are then removed. A Hough transform is performed for transforming the moving target indication data from the distance-time coordinate to the slope-intercept coordinate. Preferably, the associated moving target indication data is displayed in an animated display.
To eliminate the moving target data detected from unwanted objects or noise source, a threshold number of the superposed points can be predetermined, and the superposed points are removed from consideration when a number thereof is smaller than the threshold number. In one embodiment, the identified route is deduced from Doppler shift and route direction and speed along and the entities traveling along the identified route are travels at a constant speed.
When a plurality of entities traveling along a route, similar to the above method, a route of interest is identified, a plurality of moving target indication data is detected, and only the moving target indication data detected from the entities traveling within a proximity of the identified route are selected. The distance of the selected moving target indication data along the identified route in each instant of time is calculated. A Hough transform is then performed on the distance data to create a plurality of accumulated cells in a space of the Hough transform in response to a plurality sets of co-linear distance data, wherein each accumulated cell has an intensity proportional to the number of the co-linear distance data of the corresponding set. The moving target indication data corresponding to each entity moving at constant ground speed along the identified route even if curved can thus be dereferenced.
Similarly, to eliminate the noise and data obtained from unwanted source, a threshold intensity is predetermined, and the accumulation cells with an intensity smaller than the threshold intensity are removed from consideration. In this case there have not been enough consistent MTI observations to support the conclusion that there is a track.
In addition to distance characteristic, the present invention can also be applied to associate the moving target indication data according to other physical characteristics such as phase, directional angle, and speed. The method comprises the following steps. A route is identified, a plurality of moving target indication data is detected, and the moving target indication data obtained from entities traveling within a proximity of the identified route are selected. A physical characteristic from the selected moving target indication data at each instant of time is derived. The Hough transform is transformed on the physical characteristic to create an accumulation array. The moving target indication data corresponding to the accumulation array can thus be associated.